The present invention relates to magnetizing dilute magnetic semiconductors, and particularly to magnetizing dilute magnetic semiconductors in either thin-film or bulk form, by means of an injected current of spin-polarized electrical carriers.
Dilute magnetic semiconductors (DMS) are a class of compounds formed by alloying a semiconductor (typically a IIB-VIB compound such as ZnSe or CdTe) with a transition metal (such as Cr, Mn, Fe, Co or Ni) substituted for the group IIB ion. The exchange coupling between the magnetic transition metal ion and the conduction carriers leads to dramatically enhanced magnetic phenomena, such as a large Faraday effect, large magneto-resistance and exciton g-factors of approximately 100. However, in order to realize these effects, the system of ions and carriers must possess a net magnetic polarization. A net magnetic polarization of a DMS material is typically achieved by magnetizing the DMS material.
Previous approaches to magnetizing a DMS material are based upon the application of a magnetic field to the DMS material. Basically, there are three prior art approaches to magnetizing a DMS material, as shown in prior art FIGS. 1-3.
FIG. 1 shows a system for performing a Faraday rotation experiment on a bulk dilute magnetic semiconductor (DMS) material 11 immersed in an external magnetic field (having a magnetic field direction H) to magnetize the DMS material 11. Light is changed to plane-polarized light by a polarizer 13 before that light propagates in a direction parallel to the magnetic field direction H through the magnetized DMS material 11 to produce a Faraday effect. Thus, as the plane-polarized light passes through the DMS material 11, the plane of polarization of the light is rotated by an angle of rotation proportional to the Verdet constant of the DMS material 11, the amount of magnetization of the DMS material 11 and the optical path length through the DMS material 11. The amount that the plane of polarization of light is rotated is then used to measure the magnetization of the DMS material 11.
In FIG. 1 the external magnetic field applied to the DMS material 11 is shown as being applied by a solenoid 15, which is powered by a voltage source 17. However, such an external magnetic field could also be provided by an electromagnet, a superconducting magnet or a permanent magnet.
FIG. 2 shows another system for generating the Faraday effect in a DMS material. In this system, a bulk DMS material 19 is sandwiched between a ground plane 21 and a strip line 23 of a strip line configuration. Radio frequency (RF) current from an RF source 25 flows through a coaxial cable 27 and down the strip line 23. The RF current, which is relatively large in amplitude, propagates down the strip line 23 in the direction K polarized such that the h or magnetic field vector of the RF current is perpendicular to the DMS material 19. As a result, this large RF current flowing through the strip line magnetizes the DMS material 19.
Light from a laser 29 is then propagated through a polarizer 31 to cause the light to become plane-polarized. This plane-polarized light from the polarizer 31 passes through the magnetized DMS material 19 parallel to the h vector which causes its plane of polarization to be rotated. An analyzer 33 intercepts the light from the DMS material 19 to measure the angle through which the plane of polarization of the light has been rotated. The amount of rotation of the plane of polarization is then used to measure the magnetization of the DMS material.
The system of FIG. 2 is described more fully in the article "Frequency-dependent Faraday Rotation in CdMnTe", by M. A. Butler et al., Appl. Phys. Lett., Vol. 49 (17), pp. 1053-1055, Oct. 27, 1986.
In each of the systems of FIGS. 1 and 2, the amount of rotation of the plane of polarization of light passed through a magnetized DMS material is used to measure the magnitude of the magnetization in that DMS material. Furthermore, in each of the systems of FIGS. 1 and 2 an external magnetic field is required to magnetize the DMS material. In the system of FIG. 1, that external magnetic field can be developed by a solenoid, an electromagnet, a superconducting magnet or a permanent magnet. On the other hand, in the system of FIG. 2 that external magnetic field results from the flow of RF current through a strip line.
FIG. 3 shows a system in which a DMS material 37 is not magnetized by an external field, but rather by an incident laser beam with either a right- or left-handed circular polarization. Thus, there is a photo-induced magnetization of the DMS material 37. More specifically, linearly polarized light from a laser 39 is propagated through a quarter wave plate 41 to develop circularly polarized light with either a right- or left-handed circular polarization. This circularly polarized light passes through a pickup loop 43 of a SQUID (superconducting quantum interference device) magnetometer 45 and is incident to the DMS material 37 in the direction normal to the plane of the SQUID pickup loop 43. The circularly polarized light excites carriers in the DMS material 37 of predominately one spin state -either spin up or spin down. As a result, the DMS material 37 becomes spin-polarized or magnetized. The SQUID magnetometer 45 is used to detect the net magnetic moment (or degree of magnetization) induced in DMS material 37 by the incident circularly polarized light.
The system of FIG. 3 is described more fully in the article "Low-Temperature Magnetic Spectroscopy of a Dilute Magnetic Semiconductor", by D. D. Awschalom et al., Physical Review Letters, Vol. 58, No. 8, pp. 812-815, Feb. 23, 1987.
It should be noted that the system of FIG. 3 requires a separate laser 39 and a quarter wave plate 41 to magnetize the DMS material 37.